Current Transformers Tests - Frequency 60 or 50 Hz?

If the test frequency is at power frequency, accordingly the voltage will change by a facor.50/60=0.83,may be lower to 0.83 test voltage comparing standard.If the frequecy is so high, no need to think about it.I'm not a expert of test.Just give some oppinion.Hope this could help and would be right.

the capacitance as we know depends on frequency. and the dissipation factor or tan delta as popularly known also depends on frequency bearing the formula

dissipation factor = i/(wRC) where w = 2*pi*f where pi = 22/7 and f = frequency.

as can be inferred,

higher the frequency, lower the dissipation factor. the corresponding multiplication factor can be got by redistributing the terms on either side.

coming to the fact of excitation curve, it is a plot of the magnetising current and the voltage which is determined to find the knee point voltage or the voltage at which the core saturates. the magnetising current depends on frequency (may be proportional to the square of it). check out with an expert before coming to any conclusion. sometimes a higher frequency can cause overfluxing in the core.

To perform routine test on the 245 kV C.T., all test (except accuracy test) could be made at 60 Hz, without voltage correction. (For dielectric test in a C.T. there is not correction between 50 Hz and 60 Hz).

For accuracy test, the burden of the CT has to be increased in the proportion 60/50 (1.2), because, at 60 Hz, the section of the core for a given load is 1.2 times lower than at 50 Hz.

And to obtain the excitation curve, if you have the dimensions of the core, and the number of wires, you can obtain the relation B=f(H) without problem at 50Hz or 60 Hz. With the primary open, you only need to feed the secondary core and measure the voltage and current, and apply the approximate formula:

E = 2 * (f/50) * Ns * Bpeak * Sb *10^-6

where E: volts (rms)

f: frequency

Ns: secondary wires

Sb Section of the core [cm2]

Bpeak: induction [Gauss]

Ho*Lm = Ns*Io

where Ho: magnetic field strength [A/m]

Lm: midline of the core [cm]

Io: excitation Current [A]

Best Regards

None of the standards provide formulas relating to changes of test values if the equipment is to be tested at a different frequency than the nameplate frequency.

You only alternative is to discuss the situation the situation with your client and come to an understanding. Generally speaking:

a. The power frequency withstand test should and would likely remain the same - at 50 or 60 Hz.

b. The same basically applies to the PD or FRI test - same test voltage for 50 or 60 Hz.

c. The same applies to the C&DF test, . . . same test voltage and the same expected DF/PF values regardless of test frequency - 50 - 60 Hz. NOTE the DF values of typical of GOOD oil-paper insulation remains basically the same at 25 / 50 / 100 Hz. NO, it can not be considered as a RC combination with obvious frequency corrections.

d. The induced test (inter turn test) can be carried out at a range of frequencies, . . typically second harmonic, third harmonic or 400 Hz. It is the TEST VOLTAGE that is critical and needs to be determined.

e. The accuracy test needs to be carried out at the specified frequency and specified burden. This does not require large amounts of power and could be powered from an electronic source. As an alternative, the client may accept a test using a >CT Tester<.

f. The excitation test needs to be carried out at the specified frequency. This test also does not require much power to conduct. Alternatively the test results from a >Relaying CT Tester< may be acceptable by the client. NOTE that the saturated voltage value from this test will determine the test voltage of the induced test.

Oleh